Reduced quantities:

In practice, to describe a particular type of quantity, we often find that working with a "reduced quantity" appears to work:
- the single-particle potential (KS) for the ground state density (instead of the full many-body Hamiltonian)
- a two-point kernel for the density-density response in TDDFT (instead of the four-point BSE one)
etc.

This defines the term "reduced quantity" for the purposes of this discussion.